Usable as a charging roller for uniformly electrically charging a surface of a photo receptor body in an image forming apparatus is a semiconductive roller which is produced, for example, by injecting a semiconductive rubber composition into a predetermined press mold to mold the rubber composition into a tubular body and crosslink a rubber component of the rubber composition through press-crosslinking, and inserting a shaft such as of a metal into a center through-hole of the tubular body.
With recent development and prevalence of compact and less expensive laser printers for use in small offices and for personal use, there is a demand for easy production and cost reduction of semiconductive rollers such as charging rollers to be incorporated in the laser printers.
The rubber composition is generally imparted with a semiconductive property, for example, by using an ion-conductive rubber as the rubber component, by blending an ion-conductive agent such as an ion-conductive salt to impart the composition with ion conductivity, or by blending an electrically conductive filler such as carbon black or metal powder to impart the composition with electron conductivity.
In the former case, however, the ion-conductive rubber and the ion-conductive agent are very expensive and difficult to obtain and handle. This reduces the productivity of the semiconductive rollers, and increases the production costs of the semiconductive rollers.
In the latter case, on the other hand, the rubber composition per se can be prepared at lower costs by blending a general purpose electrically conductive filler with a general purpose rubber component.
However, it is difficult to homogenously disperse the electrically conductive filler in the rubber composition, so that there are variations in the amount of the electrically conductive filler per unit volume of the outer peripheral surface of the semiconductive roller as measured in a circumferential direction and a widthwise direction of the semiconductive roller. Accordingly, the electrical resistance on the outer peripheral surface of the single semiconductive roller is not constant with variations. If the semiconductive roller suffering from the variations in the electrical resistance on the outer peripheral surface thereof is used as the charging roller, for example, it is impossible to uniformly electrically charge the surface of the photoreceptor body, resulting in defective image formation such as uneven image density.
Where the outer peripheral surface of the semiconductive roller is covered with a coating film, the variations in electrical resistance can be accommodated. Therefore, the surface of the photoreceptor body can be uniformly electrically charged, thereby suppressing the defective image formation such as uneven image density.
In addition, when the semiconductive roller is used as the charging roller or the like in direct contact with the photoreceptor body, the image formation is prevented from being adversely influenced by contamination of the photoreceptor body with a component bleeding or blooming on the outer peripheral surface from the inside of the semiconductive roller. Further, additives such as silica added to a toner for improvement of the fluidity and the electrical conductivity of the toner are prevented from being accumulated on the outer peripheral surface of the semiconductive roller. This also prevents the adverse influence on the image formation (JP-3449726-B and the like).
The coating film is generally formed by applying a coating material liquid onto the outer peripheral surface of the semiconductive roller through a coating process such as a spraying method or a dipping method, and then drying the coating liquid. Therefore, the coating film is liable to suffer from contamination with dust and other foreign matter, uneven thickness and other defects during the coating process.
In addition, the coating film formation technique, which is an established technique, has little room for improvement. Therefore, it is difficult to significantly reduce the incidence of defects (defect percentage) as compared with the current technique. This may also reduce the yield and the productivity of the semiconductive roller, and increases the production costs.
Where the ion-conductive rubber composition or the electron-conductive rubber composition is used, various types of press molds for the press-crosslinking of the rubber composition should be always prepared for different laser printer products produced by different makers or the same maker, i.e., for different outer diameters, different widths and different shaft diameters of semiconductive rollers.
Therefore, great equipment investment is required for the press molds. This may reduce the productivity of the semiconductive roller, and increase production costs.